In this episode of the DustSafetyScience Podcast, we interview Stephen Kasik, technical sales engineer at Osprey Corporation, about the use of rotary vacuum drum filters in industries handling combustible dust.
Dr. Chris Cloney met Stephen and the Osprey team during a recent trip to the U.S. He visited their Atlanta facility, where they manufacture their rotary drum filters. They showed him how they have upgraded their systems to be compliant with combustible dust safety and removed some of the restrictions that have traditionally been put on these systems.
In this interview, Stephen answers the following questions:
- What is a rotary vacuum drum filter?
- How are they different from traditional baghouse assemblies?
- How do they address combustible dust hazards?
- Has any testing or analysis been done on these systems?
- What are the benefits of a rotary drum vacuum system?
- What industries are currently having success with these sort of systems?
What Is A Rotary Vacuum Drum Filter?
Stephen explained that a rotary vacuum drum filter is essentially a dust collector, but it differs from a baghouse in several aspects. It is a perforated, horizontally-mounted metal cage covered with filter media and enclosed in a steel house. Incoming material is collected onto the filter cloth while the drum rotates and clean air is sucked out through the center. Vacuum nozzles sit atop the drum and clean the filter media to prevent dust from building up.
Once the clean air is exhausted through the center of the rotating drum, filter stages can be put on the clean air to increase the efficiency: moving from merv 10 through to higher efficiency to get breathable air back out.
There is a time-lapsed video of the Blue Sky filter assembly that shows how the device is put together.
How Are They Different From Traditional Baghouse Assemblies?
The main difference between the traditional baghouse assemblies and the rotary drum filter is that the dust accumulating on the filter surface is being actively cleaned by the nozzles. With the baghouse, material is collected on the bottom of the enclosure. With the rotary drum filter, the material is being discharged off the line from the filter.
The highest efficiency rating on the drum section is going to be a number of 10. The downstream passive stages, which are a cassette-type filter element, are used to achieve the higher efficiency ratings of 14 HEPA quality air. In baghouses, those efficiency ratings can be achieved on that initial stage.
How Do Rotary Vacuum Drum Filters Address Combustible Dust Hazards?
Osprey’s patented Blue Sky filters achieve high HEPA quality filtration. They are also designed to offer protection from explosion hazards by addressing some of the five components of the dust explosion pentagon.
The first is ignition sources. On older equipment, lights were on the inside of the drum filter stage or a gear reducer was mounted inside of the enclosure. Osprey removed ignition sources by having all electrical components on the outside. Anything inside the enclosure is rated using ATEX systems.
The filter equipment is considered as part of the overall dust collection system. Osprey has considered keeping the MEC at 25% or below during operating conditions. The inlets for the incoming process air duct have been designed to be above the conveying velocities of the particular combustible dust being handled. The company has also gone through engineering designs to eliminate horizontal surfaces that can collect that dust.
NFPA 654 recommends that the ducts be designed so that that that dust or the air conveying the dust is moving at 4000 feet per minute. Osprey generally sizes their ducts so that the minimum speed is 4500 fpm, but depending on the system design, they may need to be sized for 6000 fpm.
Once the dust comes into the entrainment, it’s kept high enough so the dust is still in suspension. All the walls are smooth and curved to prevent dust from building up in that enclosure. It is all settling out on the filter instead. This system is different from the basic operating principle for a baghouse, which will inherently have a dust cloud around the filters each time it’s pulsed.
Rotary vacuum drum filter systems keep the dust in suspension inside of the drum enclosure so that it can be captured onto the drum itself. If material is falling out or if there are dead spots in the filter, there is no opportunity to collect it onto the filter media and then remove it with the vacuum system. The suction is kept high enough that you’re always cleaning a filter and not enclosures, and never having dust concentration above the MEC.
Stephen explained that the list of considerations even extends to the drum rotation. The European directive is that if you rotate the circumvential velocities below one meter second, there is no risk for a spark. Osprey designed the rotating drum so that there is no risk of creating a spark via metal on metal contact.
Baghouse systems remove dust by discharging it with the reverse pulse jet or reverse airflow. Once that material is discharged from the bags, it can be collected in a barrel. Osprey’s working principle is to actively clean the drum. The system is pulling the dust from the surface of the drum using a high pressure blower fan and the static pressures are designed to keep the conveying velocity above 4500 feet per minute.
“Some design considerations that we’ve looked at for this cleaning fan is related to the ATEX certification,” Stephen said.
Another important element is that the cyclone handling the dust coming from the discharge of the stripper or nozzle fan blower has been sized so that it can be installed indoors without explosion venting, making it suitable for a wider range of installation settings.
Has Any Testing Or Analysis Been Done On These Systems?
Stephen explained that internally, Osprey will do different assessments that relate to ATEX or NFPA.
“We’ll do ignition assessments or hazard analysis,” he said. “We’ll do dust property analysis. On the ATEX side, we typically deal with risk analysis and then the larger part of that is the risk assessment. We’ll do zone calculations for each stage of the filter. And then beyond that, we also have worked with some third party companies to validate those assessments and validate our approach to dealing with combustible dust.”
What Are The Benefits Of a Rotary Drum Vacuum System?
“Where this system does not require explosion, venting can be installed indoors,” Stephen said. “We’ll see it installed next to production lines. And we’ve done that with the approach that we’ve we’ve taken and the methodology and the risk-based and performance-based design approach that we’ve taken with the system.”
In one study, Osprey took a worst-case scenario, which would be a power outage while the rotary drum filter was operating. They simulated power loss by hitting the E-Stop and then collected all the dust that had entered and been entrained onto the filter media. They found that it made it met required goals with regards to the MEC.
What Industries Are Currently Having Success With These Systems?
Stephen said that Osprey’s industries are disposable hygienics, such as baby care, femine care, adult incontinence. These processes rely on moving air to form the product. Different process fans may form one layer of the diaper while others may hold the diaper to a vacuum belt. Due to the material composition, the process creates a lot of dust.
“We also provide drum filters for paper-converting industries, so tissue and paper towel. There is some involvement in plastics and also in agriculture. There are applications in those markets for this type of technology.”
He said that in certain applications, they can take collected dust from the nozzle fan, reclaim the materials, and reintroduce them back into the product, which can save money on more expensive materials.
“Beyond that, the filtration level that we can achieve with HEPA stages is beneficial for energy savings,” he explained.
Conclusion
Stephen concluded the interview by recommending that all facilities carry out a dust hazard analysis, even if their process equipment is designed for safety and they’ve never had an incident.
“That doesn’t mean there won’t be an incident,” he said. “If you follow the NFPA flow charts for DHAs or or look at the European standards that determine performance level, you can look at whether those events will be catastrophic or not, and quickly see just how critical it is to at least start that dust hazard analysis and and start making progress to protect against combustible dust accumulation and that risk in your facility.”
If you would like to discuss further, leave your thoughts in the comments section below.
You can also reach Stephen Kasik directly:
Email: Stephen.kasik@ospreyfilters.
Website: https://www.ospreyfilters.com/contact/
If you have questions about the contents of this or any other podcast episode, you can go to our ‘Questions from the Community’ page and submit a text message or video recording. We will then bring someone on to answer these questions in a future episode.
Resources Mentioned
DustSafetyScience:
Combustible Dust Incident Database
DustSafetyScience Podcast
Questions from the Community
Companies:
Osprey Corporation
Standards:
NFPA 654
Videos:
Blue Sky Filter Assembly
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DSS051: Use of Rotary Vacuum Drum Filters in Industries Handling Combustible Dust With Stephen Kasik